1. Field of the Invention
The present invention relates to an ultrasound flow measurement apparatus and, more particularly, to an ultrasound high velocity flow measurement apparatus using time domain correlation.
2. Background Information
A primary use of ultrasound high velocity flow measurement apparatus is for blood flow velocity measurement and two dimensional mapping of blood flow velocity. One conventional apparatus is known as an ultrasonic Doppler velocimeter. The Doppler velocimeter transducer emits a continuous or pulsed beam of ultrasound generally in the range of 1 to 10 MHz in peripheral and deep-lying vessels. The frequency of the returned signal, backscattered from moving red blood cells, is different from the frequency of the signal transmitted. The difference in frequency is known as the Doppler shift frequency, .DELTA.f, given by: ##EQU1## where f is the transmitted frequency, v is the velocity of blood, x is the angle between the ultrasound beam and flow direction, and c is the velocity of sound in tissue.
Because of the random motion of the blood particles within the flow and because of the velocity variation across the vessel, the Doppler signal has a spectrum with a finite width rather than a single frequency. The method of measurement utilizing the Doppler methodology is sensitive to velocity components that are parallel to the ultrasound beam but are relatively insensitive to flow components that are perpendicular to the ultrasound beam.
Another conventional apparatus used in performing the blood flow velocity measurement and profiling is known as a cross-correlation velocimeter. Ordinarily, in the conventional cross-correlation velocimeter, a single pulse is transmitted to a blood vessel from a transducer and the echoes are received before a second pulse is transmitted. Normalized cross-correlation is then used to determine a time shift (t.sub.2 -t.sub.1) of a segment of the first echo with respect to a similar segment of a second echo of the second transmitted pulse. The magnitude of the velocity v of the blood at the distance corresponding to the penetration depth of a relevant segment of the first pulse is given by the following expression: ##EQU2## where c is the velocity of sound in the medium, T.sub.PR is a pulse repetition (PR) interval and x is the angle that the ultrasound beam makes with the blood vessel. This is shown in FIG. 1. The time difference (t.sub.2 -t.sub.1) is determined by cross-correlating segments of two consecutive signals which carry the flow information. The pulse repetition interval is set by the depth at which the vessel is located in the body. Therefore, the larger the depth, the larger the pulse repetition interval. On the other hand, equation (1) illustrates that for a given (t.sub.2 -t.sub.1), the larger T.sub.PR the smaller the velocity v that can be measured. Furthermore, when the velocity v is high, the larger T.sub.PR increases the chances for the signals from two successive signals to be de-correlated. This de-correlation results because either during the time T.sub.PR scatterers flow out of the ultrasound beam, or in the case of turbulent flow, due to the random nature of the flow, the scatterers change their relative position and therefore, the signal from two successive signals become de-correlated.
Thus, the use of a conventional cross-correlation scheme for measuring high velocities at large depths presents two conflicting requirements. One requirement is a large pulse repetition interval so that the echoes can be separated between two successive signals. The second requirement is a small pulse repetition interval so that the signals remain correlated.
The issue of de-correlation of signals is rather complex. De-correlation depends on the time repetition interval and the flow velocity. Beam geometry and diameter further play important roles in determining the de-correlation time.
For a Gaussian beam with full width of w, the maximum measurable velocity at a pulse repetition time of T.sub.PR is: EQU w/(3.5 T.sub.PR sin (x))
As shown in FIG. 2, when pulse repetition time of 200 microseconds is used, the maximum depth of the vessel has to be less than 150 mm inside the body. With this repetition time, a beam diameter of 1 mm yields a maximum measurable flow velocity of 2 m/s for a vessel that intersects the ultrasound beam at 45.degree.. In cardiology or peripheral vascular applications of ultrasound, however, situations exist where measurements must be made with flow jets having velocities in excess of 10 m/s. Neither Doppler nor conventional cross-correlation schemes are capable of measuring such velocities.
The conventional cross-correlation velocimeter discussed above utilizes single, or uncoded, pulses. Coded pulses, or frequency modulated pulses with frequencies that increase or decrease with time, however can be used in ultrasound apparatus. Coded pulses historically have been used in radar and acoustic imaging systems in order to increase the signal-to-noise ratio. In the general radar system, a ramp-up coded pulse, which is a frequency modulated pulse that has a frequency increase with time, or a ramp-down coded pulse, which is a frequency modulated pulse that has a frequency decrease with time, is transmitted to a target, and a resultant reflected signal is subsequently decoded utilizing a ramp-down or a ramp-up, respectively, decoding matched filter. Utilizing coded pulses increases the signal-to-noise ratio because a matched filter can suppress all signals except the appropriately coded echoes.